Hearing is impaired in more than 10% of the human population. Despite significant progress in neonatal regeneration of mammalian cochlear hair cells (HCs), such regeneration in adults has proved extremely difficult. While gamma secretase inhibitors have shown some promise in regenerating noise-damaged auditory HCs in adult mice, no drugs have been proven effective for auditory HC regeneration in adult humans. Interestingly, we recently demonstrated that supporting cells (which surround hair cells) can be converted to HCs in mature cochleae through combined manipulation of two key genes, one of which (p27Kip1 or p27) is inactivated and one of which (Atoh1) is activated. These findings led us to screen for small-molecule inhibitors of p27 and to characterize them in cell lines. We propose to test the inhibitory effects of these lead compounds in cochlear explants and in vivo in adult wild-type and transgenic mice, with or without noise damage. These exploratory studies will provide the key proof of concept for using small-molecule inhibitors of p27, together with small-molecule activators of Atoh1, to regenerate damaged auditory HCs in adult mammals. The final lead compounds identified here will advance to the drug development pipeline for optimization, selection, and preclinical safety analysis, and eventually to clinical trials for HC regeneration in humans. These studies may lead to a breakthrough in the treatment of hearing loss caused by noise, antibiotics, chemotherapy, or age.